OhrR Is a Repressor of ohrA , a Key Organic Hydroperoxide Resistance Determinant in Bacillus subtilis

Abstract

Bacillus subtilis displays a complex adaptive response to the presence of reactive oxygen species. To date, most proteins that protect against reactive oxygen species are members of the peroxide-inducible PerR and B regulons. We investigated the function of two B. subtilis homologs of the Xanthomonas campestris organic hydroperoxide resistance (ohr) gene. Mutational analyses indicate that both ohrA and ohrB contribute to organic peroxide resistance in B. subtilis, with the OhrA protein playing the more important role in growing cells. Expression of ohrA, but not ohrB, is strongly and specifically induced by organic peroxides. Regulation of ohrA requires the convergently transcribed gene, ohrR, which encodes a member of the MarR family of transcriptional repressors. In an ohrR mutant, ohrA expression is constitutive, whereas expression of the neighboring ohrB gene is unaffected. Selection for mutant strains that are derepressed for ohrA transcription identifies a perfect inverted repeat sequence that is required for OhrR-mediated regulation and likely defines an OhrR binding site. Thus, B. subtilis contains at least three regulons ( B , PerR, and OhrR) that contribute to peroxide stress responses.

Regulation of inducible peroxide stress responses

“…Like other general stress response genes, ohrB is induced by heat shock, entry into stationary phase and various oxidants and organic alcohols (Volker et al, 1998). In contrast, ohrA is selectively induced by organic peroxides (Fuangthong et al, 2001). Regulation of ohrA is mediated by OhrR, encoded by a convergent gene.…”

“…OhrR binds to a pair of inverted repeat sequences overlapping the ohrA promoter site (Fig. 1C) and thereby blocks transcription initiation (Fuangthong et al, 2001;.…”

“…Oxidation apparently affects one or more Cys residues, postulated to include residues functioning as metal ligands, but the molecular details are not yet clear. C. The OhrR protein is shown bound to its target site as defined for B. subtilis (Fuangthong et al, 2001). Oxidation of OhrR upon exposure to organic peroxides leads to oxidation of the single cysteine residue (Cys-15), leading to the formation of a Cys-SOH (and Cys-SO 2 H) derivative and a concomitant loss of DNA-binding activity .…”

The OhrR repressor senses organic hydroperoxides by reversible formation of a cysteine-sulfenic acid derivative

“…Interestingly, a second organic peroxide resistance gene, ohrB, is under control of B (11). The role of Ohr proteins in organic peroxide resistance has now been demonstrated in Xanthomonas campestris, Pseudomonas aeruginosa, Enterococcus faecalis, Actinobacillus pleuropneumoniae, and B. subtilis (10,(12)(13)(14)(15)(16). In many organisms, ohr genes are flanked by genes encoding OhrR homologs (9).…”

“…The general stress response, coordinated by the B protein, includes paralogs of many of these stress proteins including two additional catalases (KatB and KatX) and the MrgA homolog, Dps. OhrR is representative of a recently identified class of peroxide sensors and acts as a repressor of the ohrA organic peroxide resistance gene (9,10). Interestingly, a second organic peroxide resistance gene, ohrB, is under control of B (11).…”